The presidential candidates turned their attention to Wisconsin, site of the primary season's best big contest. Gov. Scott Walker endorsed Sen. Ted Cruz, who in turn launched another round of criticism against Donald Trump. But recent polling shows that Trump and Ohio Gov. John Kasich are in a statistical tie. Gwen Ifill reports.

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GWEN IFILL:

A crucial endorsement, a criminal charge, a critical contest, it was all the stuff of another long and sometimes strange day in the presidential campaign of 2016.

All five major presidential candidates descended on Wisconsin today, the site of the primary season's next big contest. Governor Scott Walker, who briefly sought the Republican nomination himself last year, delivered his in-demand endorsement to one of the men attempting to knock off front-runner Donald Trump.

GOV. SCOTT WALKER (R), Wisconsin: I just really decided, after all these years of the Obama-Clinton failures, that it's time that we elect a strong new leader. And I have chosen to endorse Ted Cruz. Ted Cruz is the best-positioned by far to both win the nomination of the Republican Party and to then go on and defeat Hillary Clinton in the fall this year.

GWEN IFILL:

Cruz, in turn, launched another round of criticism at Trump, this time pegged to news that Trump campaign manager Corey Lewandowski was charged today with assaulting a reporter after a press conference earlier this month.

SEN. TED CRUZ (R-TX), Republican Presidential Candidate: When you have a campaign that is built on personal insults, on attacks, and now physical violence, that has no place in a political campaign and has no place in our democracy. And I think it is a really unfortunate development, but I do think it helps clarify for the voters what the Trump campaign is all about.

GWEN IFILL:

Trump's campaign maintained that Lewandowski is absolutely innocent of this charge and denied that a tape released by the Jupiter, Florida, Police Department showed him grabbing reporter Michelle Fields.

Later in the day, on the way to a town hall in House Speaker Paul Ryan's hometown of Janesville, Trump responded to questions about the charges.

DONALD TRUMP (R), Republican Presidential Candidate: If you look at her, my look and according to a lot of people, she's grabbing at me, and he's acting as an intermediary and trying to block her from doing that.

The news conference was over. It was done. It was finished, and she was running up and grabbing and asking questions. And she wasn't supposed to be doing that.

GWEN IFILL:

Recent Wisconsin polling shows Trump, Cruz and Ohio Governor John Kasich in a statistical tie. Kasich, who held a town hall in Waukesha this afternoon, says he is the strongest candidate for the fall.

GOV. JOHN KASICH (R-OH), Republican Presidential Candidate: I'm the only one that consistently beats Hillary Clinton. Nobody else consistently beats her. I beat her by 11 points in the last poll. So, that's probably going to matter: Who is it that can win? I think that's why we're having a primaries.

GWEN IFILL:

Democrats Hillary Clinton and Bernie Sanders are also in a tight race. Clinton focused today on gun violence prevention in Milwaukee, a city that has seen increasing rates of gun deaths.

HILLARY CLINTON (D), Democratic Presidential Candidate: This needs to be a voting issue, not number 20 on the list, but number one the list, especially in zip codes like this one.

(CHEERING AND APPLAUSE)

HILLARY CLINTON:

Every child deserves to have a healthy, happy life, regardless of the zip code he or she lives in. And so let's be committed to doing everything we can, in our own ways, to end this epidemic.

GWEN IFILL:

In Appleton, Wisconsin, Sanders' increasingly sharp criticism of Clinton escalated again today, as he seeks to paint her as out of touch.